Previously, coating type magnetic recording media have been prevailingly employed. Such media are produced by coating on a non-magnetic support a magnetic coating composition prepared by kneading and dispersing a powdery magnetic material. Examples of such materials include ferromagnetic oxide powders with specific examples including .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, Berthollide compounds constituted with .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, CrO.sub.2 and the like, ferromagnetic alloy powders or so on. These materials are combined with organic binders such as vinyl chloride-vinyl acetate copolymers, styrene-butadiene copolymers, epoxy resins, polyurethane resins or so on in an organic solvent. The composition is then dried to form a magnetic layer. In recent years, the demand for high density recording has increased. Accordingly, the so-called nonbinder type magnetic recording media, which contain no binders in their magnetic recording layers, have attracted a great deal of attention. The magnetic recording layers of such media are comprised of thin films of ferromagnetic metals formed by the vapor deposition techniques such as vacuum evaporation, sputtering, ion plating or like techniques, or the metal plating techniques such as electroplating, nonelectrode plating or like techniques. Various efforts to make the use of such media practical have been made.
Conventional magnetic recording media have principally utilized as magnetic materials metal oxides having saturation magnetizations smaller than those of ferromagnetic metals. Therefore, the means for reducing the thicknesses of recording materials of the coating type for the purpose of heightening their recording densities have been limited because the reduction of the thickness is attended by the lowering of the signal output. In addition, these media are undesirable because they are manufactured by complicated processes and also because large-sized incidental equipments for recovering solvents used in the manufacturing process or for preventing environmental pollutions have been required. On the other hand, nonbinder type of magnetic recording media contain ferromagnetic metals, which have saturation magnetizations greater than those of the above-described metal oxides, in the form of thin film in which any non-magnetic substances such as binder are not incorporated. Therefore, such media are advantageous in that they make it possible to use very thin magnetic films for high density recording. Furthermore, manufacturing processes for producing them are simple.
A magnetic recording medium to be employed in high density recording must use a magnetic substance having high coercive force. From a theoretical and experimental point of view, a reduction in thickness has been proposed. Such being the case, great expectations are held for nonbinder type magnetic recording media because it is easy to decrease their thicknesses one order of magnitude below the realizable thicknesses of the coating type of magnetic recording media. Furthermore, they possess high magnetic flux densities.
In particular, using an evaporation technique to form a mangetic recording layer is advantageous with respect to the disposal of waste solutions. Furthermore, the manufacturing process is simple, and a deposition speed of a magnetic metal film is high.
Thin films constituted with ferromagnetic metals have problem with regard to lasting high corrosive strength, abrasive strength and running stability. The magnetic recording medium and a magnetic head are in high speed relative motion as they come into contact with each other throughout the magnetic signal recording, reproducing and erasing processes. In such processes, smooth and stable running of the magnetic recording medium must be ensured. At the same time, wear or rupture thereof must not be caused by the continual contact with a magnetic head. Moreover, it is preferred that there is little or no decrease or erasure of signals recorded in the magnetic recording medium with the lapse of time; for example, generation of stains upon storage. In order to improve durability and weather resistance, various kinds of protecting layers have been proposed. However, such protecting layers are restricted in their thicknesses because a magnetic head and a magnetic recording layer are separated by a protecting layer. Therefore, if the protecting layer is thick the spacing loss increases even more. Accordingly, it is necessary to impart durability and weather resistance to the magnetic recording layer itself.
The above-described protecting layer or protecting film was generally made up of, e.g., a hard metal such as rhodium, chromium or the like, a hard inorganic substance such as WC, TiO.sub.2, CaF.sub.2 or the like, a lubricant, or a high molecular weight compound.
Attempts to impart both satisfactory running and durability characteristics to magnetic recording media by providing protecting films have been unsuccessful. A main cause of the failure consists in the generation of scratches on the surface of a magnetic recording medium. The scratches are caused by pieces of a hard metal or a hard inorganic substances from the protecting layer made up of such a hard material. The material breaks away due to weakness of binding at the interface of a thin magnetic metal film and a protecting film with a protecting film made up of a polymer or a lubricant. These factors caused a deterioration of the running characteristics and abrasion resistivity characteristics. These problems increased with the lapse of time to a considerable degree.
Japanese Patent Application (OPI) No. 33806/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a method of nitriding a magnetic layer in the neighborhood of its surface by direct current glow discharge of nitrogen gas. However, the running characteristics can not be improved by merely nitriding the surface of a magnetic layer. Furthermore, in order to create a protecting effect by converting the surface part of a magnetic layer into the nitride layer, it is necessary for the resulting metal nitride layer to have a considerable thickness. Therefore, the glow discharge processing must be continued for a long time ranging from 10 minutes to 2 hours.
Various processes are known for forming a protecting layer by oxidizing the surface of a magnetic layer. Examples of these processes include a process of oxidizing the surface of a ferromagnetic metal thin film under high temperature and high humidity. This is disclosed in Japanese Patent Publication No. 20225/73. Another process includes the steps of, in sequence, allowing a magnetic alloy thin film to come into contact with nitric acid, applying heat thereto to form the oxide layer in the surface part of the film and allowing a lubricant to permeate into the oxide layer, as disclosed in British Pat. No. 1,265,175. Another process involves forming an oxide layer by treating the surface of a magnetic alloy thin film with an aqueous solution of an inorganic oxidizing agent and an organic chelating agent and then subjecting the resulting surface to a heat treatment in the atmosphere of oxygen. However, these oxidizing processes are not desirable because it is difficult to form a uniform thin oxide layer. Furthermore, the vacuum condition must be broken after the formation of the thin film of a magnetic metal because aqueous solutions and the like are used in these processes. In addition, it is not possible to carry out these processes continuously. Therefore, it takes a long time to finish the above-described steps and further the control of processing conditions is complicated.